A monopulse antenna system includes plural antenna elements which receive the signal whose location is to be identified, and also includes various couplers which add the signals from various combinations of the antenna elements for generating sum signals, and azimuth and elevation difference signals. One type of prior-art monopulse antenna has four feed horns at the focus of a reflector, and a monopulse array antenna may have many antenna elements, beamformed to generate the desired sum and difference signals. In the context of such antennas, the terms "azimuth" and "elevation" are conventionally used, but refer to two mutually orthogonal measurements rather than to actual orientations. In radar parlance, an object "seen on the screen" or located within the antenna beam is termed a "target," regardless of whether it is being targeted or not; thus, a missile, an aircraft, birds, and unwanted returns from ground clutter are all referred to generally as targets. In a monopulse antenna system, the presence of the target is determined by the existence of a signal within the sum beam. In the presence of a target as determined from the sum beam, the elevation difference signal is normalized by dividing by the sum signal to generate values which establish the elevation angle, and the azimuth difference signal is divided by the sum signal to generate values which determine the azimuth angle. The quotients of the divisions are applied to look-up tables in order to determine the corresponding angular location within an antenna beam.
The beamwidth of an antenna is inversely related to the linear dimensions of an antenna measured in wavelengths; as the antenna gets smaller relative to the wavelength, the beamwidth gets larger. Some systems, like radar systems or missile seeking systems, track their targets by use of the main lobe of an antenna. In general, mobile devices must use small antennas, even when operated at the highest practical frequency, and the antenna thus tends to have a wide main beam, which imposes limits on the ability of a system to identify closely spaced sources, which in the case of a missile might cause the missile to home on a decoy located near the actual target, or to home on ground reflections. In the context of a ground-based search radar system, the time required to complete the scanning of a hemisphere requires that the antenna beam be relatively broad or large (have a large angular dimension). Thus, the antenna beam of the scanning radar antenna, being broad, is likely to contain a plurality of targets. The look-up tables of a monopulse antenna system cannot provide angles in the presence of multiple targets within the main beam of the antenna.
Improved monopulse target or source location is desired.